Method for avoiding interference radiation of AM transmitters used in digital transmission

ABSTRACT

A method for avoiding spurious emissions in an AM transmitter for digital transmission includes converting a digital modulation for controlling the AM transmitter into an amplitude signal and a phase-modulated RF signal. Non-linear distortions in an amplitude response and a delay-time characteristic of an amplitude branch are compensated for using a pre-equalization with inverse shapes by: measuring and storing the amplitude response and the delay-time characteristic of an amplitude branch; determining an inverse transfer function using respective shapes of the measured amplitude response and delay-time characteristic by an inverse Fourier transform; dimensioning an equalizing network for the determined inverse transfer function; and connecting the equalizing network in series upstream of the amplitude branch. An equalizing network is dimensioned for the determined inverse transfer function. The equalizing network is connected in series upstream of the RF branch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C.§371 of PCT International Application No. PCT/DE02/01315, filed Apr. 10,2002, which claims priority to German Patent Application No. 101 31849.9, filed Jun. 27, 2001. Each of these applications is herebyincorporated by reference as if set forth in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of broadcasttransmitters which will be converted from analog amplitude modulation(AM) to digital modulation as digitalization moves forward, andparticularly to a method for avoiding spurious emissions in AMtransmitters for digital transmission.

DESCRIPTION OF THE RELATED ART

In this context, the intention is for the hitherto usual transmittertypes, non-linear AM transmitters featuring an RF input (radiofrequency) and an audio input, to continue in use. The reasons for thisare as follows:

-   AM transmitters internally operate in switched mode and therefore    have efficiencies which are better by a factor of 3 than those of    linear transmitters which are otherwise usually used for digital    transmission, for example, in the case of DAB (Digital Audio    Broadcasting) and DVB (Digital Video Broadcasting). This results in    a saving of operating costs.-   it is easier to convince broadcasters to convert from analog to    digital if no great investments come up in the preliminary stages.

The digitalization of AM broadcasting is seen as the only chance topreserve these frequency ranges and the technology used therein in thelong term. For implementation purposes, the consortium “Digital RadioMondiale” was founded, see “Rundfunktechnische Mitteilungen”[Broadcasting Newsletter], 43rd year, 1999, issue 1, pages 29-35.

The use of a non-linear AM transmitter for digital modulation requires aspecial operating mode of the transmitter. The modulated digital signalis generated by two partial signals (I and Q), which are orthogonal toeach other. The I-signal (“in phase”) is modulated onto a cosineoscillation having the frequency Ft (carrier frequency). The Q-signal(“quadrature”) is modulated onto a sine oscillation having the samefrequency Ft. The sum of both modulated oscillations produces thecomplex modulated data signal (cosine 0 180 degrees, sine 90−+90degrees). The modulated I/Q-signal is shaped by filters in such a mannerthat it has exactly the prescribed curve shape with the desiredbandwidth.

For non-linear operation, it is required for the modulated I/Q-signal tobe converted in such a manner that the two signals amplitude signal(A-signal) and phase-modulated carrier signal (RF-P) result therefromwhich are suitable for proper control of the AM transmitter. Then, atthe output of the AM transmitter, the modulated I/Q-signal is generatedagain with higher power.

The modulated I/Q-signal corresponds to a Cartesian representation. TheCartesian representation is converted to a polar representation withamplitude and phase. In this manner, the amplitude signal (A-signal) isobtained to control the AM transmitter at the audio input. Aphase-modulated radio frequency (RF-P signal) is generated from theinitially resulting phase signal (P-signal). Advantageously, the RF-Psignal can also be directly obtained without the intermediate step viathe P-signal. In this manner, the signals are obtained that are requiredfor controlling the AM transmitter:

-   amplitude signal (A-signal)-   phase-modulated RF signal (RF-P signal)

The A-signal is fed into the modulator input (audio input) of the AMtransmitter, and the RF-P signal is used for HF-type control of thetransmitter. In the transmitter output stage, the two signals A & RF-Pare multiplicatively combined, forming the high frequency digital outputsignal.

Due to the required conditioning process, both the A-signal and the RF-Psignal obtain far larger bandwidths than the one the digital signaloriginally had and is intended to have again at the output of thetransmitter.

Older modulators are frequently not able to provide the increasedbandwidths (3-5 times the bandwidth in the AM mode of the transmitter)because they were not designed for this. When using only the limitedbandwidth that “older” transmitters have available in the modulatorsection, then this results in considerable out-of-band and spuriousemissions. These have the property that they have only a very smallgradient in the spectrum and therefore interfere with quite a number ofadjacent channels.

SUMMARY OF THE INVENTION

Thus, the band limitation in the amplitude branch (A-branch) and also inthe RF-branch results in spurious emissions whose shoulder distance inrelation to the spectrum of the data channel does not or only veryinadequately meet the requirements of the ITU.

In particular, the band limitation in the A-branch and also in theRF-branch causes the following transmission errors:

-   the amplitude response in the A-branch is not constant but decreases    with increasing frequency,-   phase response in the A-branch is not linear and, consequently, the    propagation time is not constant,-   the amplitude response and the delay-time characteristic of the    RF-branch are not constant,-   signal propagation times of the A-branch and of the RF-branch are    different (delay).

The different signal propagation times between the amplitude branch andthe RF-branch have to be accurately adjusted to values smaller than 1microsecond in the digital modulator. However, the adjustment works onlyif the propagation times both in the amplitude branch and in theRF-branch are constant over the frequency. However, coupled orassociated with the decrease of the amplitude response with increasingfrequency, the band limitation in both branches also causes anon-linearity of the phase, that is, non-constant propagation times.

An object of the present invention is to eliminate or reduce non-lineardistortions arising because of the band limitation in the amplitudebranch and also in the RF-branch of an AM transmitter so as to meet theITU limits during the emission of a digital signal.

The present invention provides a method for avoiding spurious emissionsin an AM transmitter for digital transmission. The method includes:converting a digital modulation for controlling the AM transmitter intoan amplitude signal and a phase-modulated RE signal; compensating fornon-linear distortions in an amplitude response and a delay-timecharacteristic of an amplitude branch using a pre-equalization withinverse shapes by:

-   -   measuring and storing the amplitude response and the delay-time        characteristic of an amplitude branch;    -   determining an inverse transfer function using respective shapes        of the measured amplitude response and delay-time characteristic        by an inverse Fourier transform;    -   dimensioning an equalizing network for the determined inverse        transfer function; and    -   connecting the equalizing network in series upstream of the        amplitude branch; adjusting a bandwidth of resonant circuits of        an RF branch by suitable selection of a quality of the resonant        circuits so as to achieve a linear shape for an amplitude        response and delay-time characteristic of the RF branch; and        compensating for a propagation time difference between the        amplitude branch and the RF branch in a digital modulator by        delaying a signal having a smaller propagation time.

Since in the RF-branch, the bandwidth is determined by the frequencylimits of the resonant circuits, in particular of the driver stage, itis possible, through suitable selection of the quality of the resonantcircuits, to adjust such a large bandwidth that the then still resultingout-of-band and spurious emissions become negligible compared to thoseresulting from other influences.

For the amplitude branch, the intention is to remedy the band limitationand the resulting distortion of the amplitude response and thedelay-time characteristic by applying a pre-equalization with an inversetransfer function to the amplitude signal to thereby achieve an optimumcompensation of the non-constant amplitude response and delay-timecharacteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart of a method for avoiding spurious emissions inAM transmitter for digital transmission.

Referring to FIG. 1, according to a method for avoiding spuriousemissions in AM transmitter for digital transmission, a digitalmodulation for controlling the AM transmitter is converted into anamplitude signal and a phase-modulated RF signal (see block 102).Non-linear distortions in an amplitude response and a delay-timecharacteristic of an amplitude branch are compensated for using apre-equalization with inverse shapes by: measuring and storing theamplitude response and the delay-time characteristic of an amplitudebranch; determining an inverse transfer function using respective shapesof the measured amplitude response and delay-time characteristic by aninverse Fourier transform; dimensioning an equalizing network for thedetermined inverse transfer function; and connecting the equalizingnetwork in series upstream of the amplitude branch (see block 104). Anequalizing network is dimensioned for the determined inverse transferfunction (see block 106). The equalizing network is connected in seriesupstream of the RF branch (see block 108).

DETAILED DESCRIPTION

The inverse shape of the transfer function is limited to the frequencyrange from 0-20 KHz. This value is determined by the fact that in theA-branch a bandwidth of about 3-5 times the audio frequency bandwidth orabout 3-5 times the bandwidth of the digital I/Q signal of 4.5 KHz (longwave and medium wave) or 5 KHz (short wave) is required and that, due tothe limitation to this range, no instability can arise. In the case thattransmission channels (two to four adjacent channels) are bundled at thetransmitter end, the numerical values mentioned for the bandwidth areincreased according to the bundling factor.

To apply this method, the curve shapes of the amplitude response and thedelay-time characteristic of the A-branch are determined with measuringtechniques and available in stored form. Then, the mathematical valuesof the transfer function for the absolute value and the phase aredetermined by numerical interpolation, the shape of the phase beingdetermined by integration over the measured shape of the propagationtime. The impulse response to the measured shapes of amplitude and phaseis determined by an inverse Fourier transform. In this manner, theimpulse response yields the coefficients for the filter for thepre-equalization of the amplitude signal so that a compensation of thenon-linearities in the A-branch is achieved.

The network used for the pre-equalization can be a filter having a FIRstructure (finite impulse response) in which the signal can be tappeddownstream of each of the stages of the chain of delay elements so thatthe tapped signals can be weighted and added up according to the desiredimpulse response.

The network with inverse shapes of amplitude, phase or propagation timeis connected in series upstream of the amplitude branch of thetransmitter.

Since the antenna matching of the transmitter acts back upon theamplitude response, the phase response and the delay-time characteristicof the A-branch, different shapes arise, depending on the matchingconditions. Therefore, these shapes are measured for usual matchingconditions and the results are stored. This makes it possible to selectthe appropriate shapes for the compensation, for example, in the case ofweather-related variations in impedance of the antenna matching. Theselection criterion for this can be provided by the measuring device forthe antenna matching which is present in the transmitter anyway andwhich, possibly, needs to be modified and adapted for that purpose. Inthis manner, the compensation is ideally adapted to the practicaltransmitter operating conditions. Through corrections of the storedcurve shapes, it is also very easily possible to control an optimum forthe achieved shoulder distance.

The compensation of the non-constant amplitude response and delay-timecharacteristic by a pre-equalization with an inverse transfer functioncan also be used for the RF-branch.

As far as the amplitude branch and the RF-branch have constantpropagation times, the propagation time difference between the twobranches can be compensated for using the simple method of delaying thesignal having the smaller propagation time.

1. A method for avoiding spurious emissions in an AM transmitter fordigital transmission, the method comprising: converting a digitalmodulation for controlling the AM transmitter into an amplitude signaland a phase-modulated RF signal; compensating for non-linear distortionsin an amplitude response and a delay-time characteristic of an amplitudebranch using a pre-equalization with inverse shapes by: measuring andstoring the amplitude response and the delay-time characteristic of anamplitude branch; determining an inverse transfer function usingrespective shapes of the measured amplitude response and delay-timecharacteristic by an inverse Fourier transform; dimensioning anequalizing network for the determined inverse transfer function; andconnecting the equalizing network in series upstream of the amplitudebranch; adjusting a bandwidth of resonant circuits of an RF branch bysuitable selection of a quality of the resonant circuits so as toachieve a linear shape for an amplitude response and delay-timecharacteristic of the RF branch; and compensating for a propagation timedifference between the amplitude branch and the RF branch in a digitalmodulator by delaying a signal having a smaller propagation time.
 2. Themethod as recited in claim 1 wherein the inverse transfer function islimited to a frequency range of 3-5 times of a bandwidth of a digitalI/Q signal so as to avoid instability and overdriving of thetransmitter.
 3. The method as recited in claim 1 further comprisingmeasuring and storing the amplitude response and the delay-timecharacteristic of the amplitude branch for different conditions for areactive antenna matching so that the pre-equalization can be optimallyadapted for weather-related variations in impedance.
 4. A method foravoiding spurious emissions in an AM transmitter for digitaltransmission, the method comprising: converting a digital modulation forcontrolling the AM transmitter into an amplitude signal and aphase-modulated RF signal; compensating for non-linear distortions in anamplitude response and a delay-time characteristic of an RF branch usinga pre-equalization with inverse shapes by: measuring and storing theamplitude response and the delay-time characteristic; determining aninverse transfer function using respective shapes of the measuredamplitude response and delay-time characteristic by an inverse Fouriertransform; dimensioning an equalizing network for the determined inversetransfer function; and connecting the equalizing network in seriesupstream of the RF branch; and measuring and storing the amplituderesponse and the delay-time characteristic for different conditions fora reactive antenna matching so that the pre-equalization can beoptimally adapted for weather-related variations in impedance.